1. Field of the Invention
This invention is an asymmetrically oriented, heat set, polymeric film which is frangible, thermally stable, optically clear, and which is substantially free of color fringes upon viewing in partially polarized light.
Such film may be bonded to materials, such as polyvinyl butyral, to form a two-ply laminated structure, which structure further may be bonded to a glass layer of a glazing structure to provide desired optical and mechanical characteristics. Preferably, the free surface of the film is coated with an abrasion-resistant coating, when used in these structures.
2. Description of the Prior Art
The use of polymeric films in glazing applications is old. The problem has been to obtain all of the desired properties in the structures made using such films.
Canadian Pat. No. 596,193 to Gore et al. is exemplary of the prior art. Such patent is directed to a safety-glass laminate having a number of layers one of which is a sheet of biaxially oriented polyethylene terephthalate. While this film possesses a number of the desired properties sought in windshield use applications, it lacks other properties, such as frangibility and freedom from color fringes or "rainbow effects," as key examples.
Other U.S. Patents of the prior art are: U.S. Pat. Nos. 2,526,728 to Burk et al., 2,837,454 to Watkins, and 3,437,552 to Bowen.
None of these patents, taken singly or in combination, suggest the specific polymeric film of this invention which, in combination with the other materials in laminate form, provides the total combined properties of frangibility, optical clarity, thermal stability, formability and the like and, at the same time, possesses the highly sought after additional, and critical in automotive glazing use areas, property of being substantially free of color fringes upon viewing transmitted polarized light. The provision of all of these properties in a single film is the touchstone of this invention.